Tragopogon dubius: Multiple introductions to North America and the formation of the New World tetraploids

Tragopogon (Asteraceae) is an evolutionary model for the study of whole-genome duplication, with two recently and repeatedly formed allopolyploids, T. mirus and T. miscellus, and many additional polyploid species. Tragopogon mirus and T. miscellus formed in western North America following the introduction of three diploids from Europe: T. dubius, T. porrifolius, and T. pratensis. Of these diploids, T. dubius is a shared parent of both tetraploids and is broadly defined and widely distributed in Eurasia. Because human-mediated intercontinental introductions may lead to hybridization with local species, and associated polyploidization, the introduction history of T. dubius from Europe to North America provides further opportunity to investigate both the extent and consequences of plant introductions. Using ITS sequence data, we show that the morphologically diverse, broadly defined T. dubius comprises a complex of at least 10 different ITS types in its native range, six of which have been introduced from Europe into North America. Significantly, although the two allotetraploid species have each formed multiple times on geographical scales from local to regional, recurrent formation is the result of repeated hybridization involving only one of these ITS subtypes. These results reinforce earlier data suggesting that not all diploid genotypes can form allopolyploids. Several entities traditionally considered part of T. dubius s.l. are now recognized as distinct species (e.g., T. lainzii), and it is likely that other distinct ITS genotypes identified here may also mark unique lineages that ultimately merit recognition as separate species. However, more intensive phylogenetic analyses involving many more samples from across the geographic range of T. dubius are required, as are detailed assessments of taxonomy, morphology, and cytology.

Automated summary

Tragopogon is an important model for the study of whole-genome duplication, and researchers have found that introduced T. dubius plants from Europe have formed multiple tetraploid species in North America,with repeated formation involving only one of the ITS subtypes. The study also suggests that many entities traditionally considered as T. dubius should be recognized as distinct species.